Unit 5 Day 4 – Transformers

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Day 4: Transformers
• Definition of a Transformer
• Transformer Windings
• Transformer Operation using
Faraday’s Law
• Step-up & Step-down Transformers
• Turns Ratio
• Examples of Transformer Use
Definition of a Transformer
• A transformer is a device for increasing
or decreasing AC voltage
• A transformer consists of two
independent coils wrapped around
a common ferrite core.
• The input winding is called the primary winding, and the
output winding is called the secondary winding
Transformer Operation
• A transformer is designed so that a change in magnetic
flux in the primary coil, passes through the secondary
coil. Energy losses through the core are minimal
• An AC voltage applied to the primary coil, produces a
ΔΦB and induces an AC voltage of the same frequency
in the secondary coil
• The voltage in the secondary coil is dependent upon the
ratio of the turns in the coils in the transformer
d B
Faraday' Law
VP  N P 
dt
d B
VS  N S 
dt
VS
NS
NS
 
or VS  VP 
VP N P
NP
Step-up & Step-down Transformers
•
If the secondary coil has more turns (loops) than the
primary, it is called a step-up transformer
•
If the secondary coil has less turns (loops) than the
primary, it is called a step-down transformer
•
Energy conservation requires that (neglecting core
losses) that the primary power equals the secondary
power.
VP  I P  VS  I S
or
IS NP

IP NS
Transformer Operation
• Transformers only work with AC current. A DC
current in the primary coil does not produce a
changing magnetic flux, therefore it does not
induce an EMF in the secondary coil.
• A voltage step applied to the primary coil, will
only induce a voltage spike in the secondary
coil.
Examples of Transformer Use
• Cell phone chargers
• Car ignition coil
• Power company HV Transformers
Examples of Transformer Use
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